This invention relates to optical grating hydrophones, and, more particularly, to a bias compensation technique for optical grating hydrophones.
An optical grating hydrophone is a device employed to convert time varying acoustic waves under water into optical signals, which can be used for detection of a sound source within the water. The hydrophone includes two optical waveguides (such as light fibers) in axial alignment with a narrow gap separating their ends. The optical grating hydrophone utilizes a pair of gratings located in the gap between the optical waveguides, which consist of equal width opaque and transparent stripes as a controllable aperture between the waveguides. When the opaque stripes of one grating coincide with the transparent stripes of the other grating the net transparent area is zero. When the transparent stripes of both of the gratings coincide, the net transparent area is at the maximum, one half the total area. The optical transmission from one waveguide to the other, therefore, varies from 0-50%. An acoustic signal received by a compliant part of the hydrophone supporting one of the gratings moves that one grating with respect to the other, which results in the modulation of the intensity of a light beam passing through the gratings. Such hydrophone systems require a static setting of the gratings relative to each other, the bias of the system, which establishes the light beam intensity when no acoustic wave is being received. This intensity is the base line against which the light beam intensity is measured to determine the characteristics of the received acoustic wave. To properly interpret the intensity modulation produced by a received acoustic wave, the bias must be within a known range. Especially in an array of hydrophones, where different biases occur from hydrophone to hydrophone as a result of manufacturing tolerances and operating parameters, some technique for bias compensation is very important. For the information produced by the intensity modulation to be useful, each hydrophone must be operating in a linear region of the transfer function which relates the received acoustic signal to the output optical signal. Mechanical adjustment of each pair of gratings to establish a common bias is especially difficult when high density gratings are used, since the small dimensions of the grating stripes require a very fine adjustment. This limits the applicability of this type of intensity modulated optical hydrophones to small arrays.